1. Field of the Invention
The invention relates generally to reverse osmosis and ultrafiltration fluid separation processes, and is applicable particularly to water desalination and purification by reverse osmosis.
2. Prior Art
In reverse osmosis a feed fluid, eg. saline water, is pumped at an elevated working pressure into a pressure vessel containing semipermeable membranes. Purified product water of greatly reduced salinity permeates across the membranes into a low pressure collector if the working pressure exceeds feed fluid osmotic pressure. Considerable excess working pressure above the feed fluid osmotic pressure is required to produce sufficient product water flux across membranes of reasonable surface area, and also to ensure sufficient dilution of the small but finite salt diffusion through the membrane which always exists when there is a concentration gradient across such membranes. For sea water whose osmotic pressure is about 25 Kg/sq. cm., typical working pressure for single stage reverse osmosis is of the order of 70 Kg/sq. cm.
While some of the feed fluid stream permeates through the membranes, the balance becomes increasingly concentrated with salt rejected by the membranes. In a continuous reverse osmosis process, a concentrate fluid stream must be exhausted from the vessel to prevent excessive salt accumulation. In sea water desalination, this concentrate fluid stream may be typically 70 per cent and sometimes as much as 90 per cent of the feed stream. The concentrate stream leaves the vessel at almost full working pressure, but before the concentrate stream is exhausted from the apparatus, it must be depressurized. The concentrate stream can be depressurized by throttling over a suitable back pressure valve to regulate the working pressure and dissipate all the pressure energy, but this energy dissipation reduces efficiency. It is known to recover some of the concentrate fluid pressure energy and two examples of energy recovery devices in reciprocating pumps are shown in U.S. Pat. No. 3,558,242, Inventor W.D. Jenkyn-Thomas, and 4,124,288, Inventor L. P. S. Wilson. In these patents, a sliding spool valve or rotary valve is used to control direction of concentrate fluid flow into and out from the expansion chamber of the feed pump, the valve being phased relative to the pump stroke to permit fluid entry and exit precisely at extreme limits of the piston stroke. This, of course, requires accurate valve timing and fast valve shift to reduce chances of hydraulic lock or system pressure loss, because movement of the valve means cannot occur while the piston means is moving. It follows that the valve timing mechanism requires close manufacturing tolerances, and thus excessive wear in the valve mechanism would likely produce severe problems.
As stated previously, the working pressure is commonly of the order of 70 Kg/sq. cm. and thus directional valve gear is subject to relatively high forces, particularly in view of the relatively poor lubricity of sea water. In the two references referred to above, directional valve shifting occurs across ports subjected to high pressure difference which subject the valves to severe erosion problems, and may impose high loading on the valve gear, tending to aggravate wear and compound the risk of pressure loss.
Furthermore, for high efficiency, concentration polarization must be controlled during reverse osmosis. Concentration polarization is the tendency for a concentration gradient to develop in the feed fluid stream with high salt concentration adjacent the membrane face. This tendency results from the bulk transport of saline feed water toward the membrane face and the accumulation of salt in the boundary layer adjacent the membrane face as less saline water permeates through the membrane, balanced by diffusion of salt back out of the boundary layer. Concentration polarization is detrimental especially with feed fluid solutions of high osmotic pressure such as sea water, because the membrane sees a higher concentration which raises the effective osmotic pressure. When concentration polarization occurs, working pressure for given product flux must be increased, product salinity will be increased, and membrane life may be impaired.
Reverse osmosis systems are typically designed to reduce concentration polarization effects by forced convection through the membrane array. It is essential that continuous feed circulation be maintained past the membranes, because even momentary stagnation of flow may cause severe concentration polarization. Whilst hydraulic accumulator devices are satisfactory in some applications to reduce pressure and flow fluctuations across the membranes during return strokes of the pump, in other applications improved forced convection can be attained by circulating a low ratio of product flow to concentrate flow across the membrane faces, or by auxiliary recirculation, or by mechanical stirring devices. However, recirculating flows past the membranes, stirring devices, etc. commonly increase considerably the complexity of the apparatus.